This invention relates to asynchronous transfer mode (xe2x80x9cATMxe2x80x9d) telecommunication devices. The invention has particular application to ATM switches. The invention provides methods and devices for detecting the loss of ATM cells in one or more cell streams passing along one or more data paths in a telecommunication device.
An ATM network is a connection-oriented packet switched network. An ATM network includes a number of ATM telecommunication devices, including ATM switches, which interconnect ATM end points via point-to-point transmission links. A xe2x80x9ccallxe2x80x9d may be set up to provide one or more virtual connections between selected endpoints. Cell streams flow along the virtual connections from one end point on the network to another. On their way, the cell streams traverse various ATM switches and other telecommunication devices.
The telecommunication devices in modern ATM networks must operate at extremely high levels of reliability at extremely high speeds. The increasing importance of real time applications such as video conferencing, video-on-demand, and the general increase in the amount of data which must be moved from place to place is placing ever higher demands on ATM telecommunications networks.
In ATM networks, information is delivered across transmission links in fixed-length cells. Each cell includes a data payload and a header which includes information about the cell and its destination. In the current version of ATM specified in Bellcore, Generic Requirements, GR-1113-CORE, xe2x80x9cAsynchronous Transfer Mode (ATM) and ATM Adaptation Layer (AAL) Protocolsxe2x80x9d, each ATM cell has 53 bytes including a 48 byte payload and a 5 byte header.
Data packaged in ATM cells can be sent over the ATM network from one end point to another. In any ATM network some loss of ATM cells may occur during normal operation. ATM uses admission control and resource reservation when a call is set up between two endpoints to provide guaranteed levels of bandwidth and other quality of service (xe2x80x9cQoSxe2x80x9d) parameters for the call. ATM networks are designed to permit ATM cells to be dropped. ATM cells can be dropped, for example, as the result of the operation of usage parameter controls (xe2x80x9cUPCxe2x80x9d) at the ingresses of ATM switches. UPC may refuse to accept cells in certain cell streams to prevent an ATM switch from becoming overloaded in a manner which could interfere with QoS guarantees for an existing call. ATM cells may also be dropped as a result of buffer overflows at queuing points in ATM switches or other telecommunication devices.
Various measures are used to evaluate the QoS provided end-to-end by an ATM call connecting two ATM end points. One aspect of QoS is the proportion of ATM packets which are dropped. For example, it may be necessary or desirable to guarantee that no more than 1 in every 107 or 108 ATM cells traversing a particular call will be dropped. Various tools and techniques are available to establish the rate at which ATM cells are dropped over an end-to-end link. Tools are also provided to test for the number of ATM cells which are dropped over a particular transmission link which connects two ATM switches. In general, however, the current ATM network technology assumes that ATM cells are never unintentionally lost internally to an ATM switch. This is a problem because ATM cells can be lost internally to ATM switches, particularly when the switches are operating at extremely high data rates.
Furthermore, malfunctions in switches can cause the loss of ATM cells. An ATM cell may be lost from a data stream if, for example, its header becomes corrupted so that the cell is no longer recognized as being part of the cell stream to which it should belong. Unintentional cell loss is a xe2x80x9csilent failurexe2x80x9d which will only be detected if an end customer complains.
It is known to detect cell loss in an ATM switch by providing ingress and egress counters for each virtual circuit passing through the switch. The ingress counters count the number of ATM cells in a virtual circuit which enter the switch and the egress counters count the number of ATM cells in the virtual circuit leaving the switch. The problem with this approach is that there is no way to know how many cells in the cell stream for a virtual circuit are buffered within the switch at any given time. At any given time, there may be thousands of cells buffered at various queuing points within an ATM switch, or other ATM telecommunication device.
A second problem with using ingress and egress counters to detect cell loss is a synchronization problem. Ingress and egress counters must be sampled at exactly the same time (typically within 1 microsecond of each other, or faster) in order to obtain meaningful results. This is typically impossible where the polling of counters is controlled by software. One could reduce the error caused by the fact that ingress and egress counters cannot be sampled at exactly the same times but providing high capacity counters and integrating over a long period of time. This, however, delays the availability of results.
Schefts, U.S. Pat. No. 5,142,653 proposes a system which includes a low capacity circular counter for each virtual connection in a switch. The counters are set up when a virtual connection is established. After the virtual connection is cleared down the loss rate for the virtual connection is calculated from the counter values. The cell loss rate is determined by the difference in cells counted by the egress and ingress counters on clear down. The disadvantage of this approach is that it only detects cell loss when a call is completed.
Tremel et al., U.S. Pat. No. 5,491,697 generates and inserts measurement cells into a cell stream. The number of measurement cells received at a reception and processing unit can be tracked as can be the total number of cells received at the reception and processing unit. The Tremel et al. device does not enable the direct detection of lost cells. Furthermore, the insertion of measurement cells decreases throughput of the network for other ATM cells.
Kawasaki et al., U.S. Pat. No. 5,878,063, adds 1 byte to each ATM cell which is received at a switch. In that 1 byte, 1 bit is allocated for a cell counting flag. The cell counting flag is set for a given period of time. A counter down stream counts the number of cells in which the flag has been set. The same count start/stop command both terminates counting and terminates the insertion of cell counting flags At the end of the counting period the system checks to see if the down stream count matches the up stream count. The Kawasaki et al. device does not deal with the problem that a certain number of cells will be buffered within a switch or other telecommunications device at any given time and does not operate continuously.
Yokoyama et al., U.S. Pat. No. 5,757,775 discloses a system for protecting against cell loss in a telecommunications system. A cell loss detector is provided in a ATM-PBX interface. Each cell reaching the cell loss detector has a sequence number. The cell loss detector watches incoming cells for discontinuities in the sequence numbers. If there is a discontinuity in the sequence numbering then the cell loss detector signals a cell loss. The sequence number used by the Yokoyama device is the 3-bit serial count sub-field of the 4-bit serial number field specified for the AAL layer of ATM cell formats according to ITU-T Recommendations I-363 and I-365.1. Consequently, the Yokoyama device will work only for AAL-1 cell streams.
There is a need for methods and apparatus for reliably detecting the unintended loss of ATM cells within ATM telecommunications devices. There is a particular need for a method and devices which can detect unintentional cell loss without tearing down virtual circuits in the switch or other telecommunications devices. Preferably the number of lost cells is detected as well as the fact that some cells have been lost. There is a need to detect the location at which ATM cells are unintentionally dropped so that suitable corrective action may be taken.
This invention provides methods and apparatus for detecting cell loss in ATM telecommunication devices in which a synchronization signal is delivered from upstream locations to downstream locations in ATM cell streams. One aspect of the invention provides a method for detecting cell loss in a ATM telecommunication device, the method comprises counting cells in a cell stream at an upstream location in a telecommunication device; counting cells in the cell stream at a downstream location in the telecommunication device; in response to a toggle event, preserving a count at the upstream location and passing a count synchronization signal in a cell in the stream; at the downstream location, receiving the cell, detecting the count synchronization signal and, in response to the count synchronization signal, preserving the down stream count; and, comparing the up stream and down stream counts.
Further aspects and advantages of the invention are described below.